group_by — Group Items by a Key

from collections import defaultdict
from typing import Callable, Iterable, TypeVar

T = TypeVar("T")
K = TypeVar("K")

def group_by(items: Iterable[T], key: Callable[[T], K]) -> dict[K, list[T]]:
    out: dict[K, list[T]] = defaultdict(list)
    for item in items:
        out[key(item)].append(item)
    return dict(out)

users = [
    {"name": "Alice", "team": "A"},
    {"name": "Bob",   "team": "B"},
    {"name": "Cara",  "team": "A"},
]
print(group_by(users, key=lambda u: u["team"]))
# {'A': [Alice, Cara], 'B': [Bob]}